Search engine crawlers evaluate the PageRank largely based on the type of links a website has (among other factors). Links are generated by promoting your website so as to attract other webpages towards your page or by generating internal links to increase the quantity of inbound links for an important page (within your website) thru cross linking.
Backlinks and inbound links are the two variants where former is a kind of reference to some other webpage while later is a reference form some other page to your page. Among other factors, the quantity and quality of these links are an important factor considered during ‘PageRank(ing)’ by search engine’s algorithms. No doubt the quality and relevance of content, structure of headings, webpage layout, keyword selection, word structing all play an important part in attracting and retaining visitors, however web ‘crawlers’ and ‘spiders’ during their webpage analysis critically evaluate these links to accordingly establish the ranking of a webpage search engines indexing system. White Hat and Black hat SEO practices are largely based on the manipulation of these link types.